Medicago, like many other legumes, has a whole genome duplication event
(aka polyploidy) within its lineage. This particular WGD event is dated to have
occurred approximately 58 million years ago and contributed significantly to the
genome we see today. The increase of duplicate genes and subsequent functional
divergence might have supported the evolution of nodulation and symbiotic nitrogen
fixation.

Extensive gene fractionation between WGD-derived homeologues has left few remnants of the event. The figure below illustrates the small synteny blocks that are typical between medicago chromosomal regions.

Significant macrosynteny has been described among legume species. Conserved blocks, sometimes as large as chromosome arms, span major
portions of the euchromatin. A given Medicago region is typically syntenic with one other Medicago region (though
usually in small blocks showing degraded synteny), four Glycine max
regions, one or two Lotus japonica regions, and three
Vitis vinifera regions (dot plots not shown).